1. Field of the Invention
The invention relates to bulk material sorting methods and apparatus, and in particular to sorting materials such as particulate recycled glass by heating a conveyed stream of particles using microwave induction heating and then discriminating for particles at different temperatures. Microwave energy, preferably in the frequency range of 0.915 to 2.450 GHz, differentially heats glass cullet particles of different compositions due to their differing thermal characteristics, dielectric strengths and loss tangents. The different compositions that are sorted can comprise glass particles having different melting temperatures such as borosilicate glass vs. pyroceramics. The heated cullet is discriminated and sorted by capturing and analyzing pixel image frames from a thermal imaging camera, including separately analyzing laterally adjacent lanes in the image frames. Thermal image data captured by the camera is used to control one or more mechanisms operable to divert materials having a detected temperature different from an average temperature of the stream of cullet in each lane, thereby separating out or concentrating predetermined materials. The invention is particularly useful for separating ceramic contaminants from recycled glass as well as metals.
2. Prior Art
Municipalities routinely collect glass for recycling; however even after substantial washing and sorting steps, the composition of the collected glass is not uniform. Processors attempting to use the recycled glass find, for example, that some of the glass particles melt at a higher temperature than others, which leads to processing problems that render the recycled material unsuitable for certain uses. Recycled glass materials comprise containers and broken glass pieces that are clear (or "flint") or colored, typically amber or green. When collected for recycling, various non-glass contaminant materials are present. Non-glass contaminants having physical properties that are distinctly different than those of the glass can be sorted out by discriminating for differences in properties. For example, metal bottle caps and the like can be removed during crushing and screening steps because metal is malleable and is flattened by crushing, but glass is frangible and breaks into small pieces that can be screened. Ferromagnetic metals can be removed magnetically. Contaminants such as organics, dirt, paper from labels and the like can be removed by washing, rinsing and filtering steps. Plastics are generally less dense than glass and can be removed by rinsing, skimming or winnowing steps.
A greater problem is confronted in attempting to sort out materials that have properties that are quite similar to those of glass. Occasional pieces of ceramics, pyroceramics (e.g., Corning Corelle ware), tempered glass (e.g., Corning Pyrex glass), stones and other materials are frequently present in the recycled cullet. These contaminants are not readily distinguishable from glass by their physical properties, which in many respects resemble the properties of glass. Most sorting techniques are not effective to detect and remove them because their physical, electrical and chemical properties are substantially the same as those of glass. Recycling authorities accordingly may instruct consumers to recycle only glass food and beverage containers, and not (for example) glassware, crystal, ceramics, plate glass, mirrors and the like. Inevitably some of these materials find their way into recycled glass.
Contaminants can cause difficulties in processing cullet into new glass. The recycling process typically includes comminuting the glass into relatively small particles that are processed in bulk. Particles that have properties that are distinct from those of the other particles represent defects in the bulk material. For example, ceramics and tempered glass have a higher melting temperature than glass, such that they may not melt completely and/or mix homogeneously in remelted glass. The result may be localized defects in molded recycled glass, clogging of spinnarettes used to make fiberglass from recycled glass, and other problems. For these reasons, recycled glass may be relegated to uses taking negligible advantage of its properties, such use as a filler in paving material. On the other hand, it is impractical or impossible to manually pre-sort cullet effectively before comminution or breakage.
Many types of materials are recycled, such as glass, plastic, paper and metals, often in mandatory programs intended to reduce waste and conserve landfill space. A municipality may require that recycled materials be sorted manually into different containers that are dumped into different receptacles upon collection. More often, there are too many categories to justify segregation of each variety of material through the process. Typically glass, plastic and metal are collected together for later sorting. Even if an attempt is made to sort by material, consumers cannot be expected to be sensitive to a difference in types of glass, and may commingle distinct materials inadvertently.
Recyclables collected in commingled collection programs need to be sorted during further processing if a relatively pure material is needed, for example to make new glass containers or the like from the recycled ones. Manual sorting is possible but tends to be prohibitively expensive, and the most careful sorting can be ineffective when much of the glass is broken. Due to the inability to sort types of glass and to separate glass and non-glass materials, recycled glass cullet often is not used to make new glass, and is substantially less valuable than material that is more pure. For example, recycled glass cullet of moderate particle size may be used as an aggregate or filler in roadway paving. Smaller particles may be used as "sand" for golf course bunkers. Although these uses are not insubstantial, the economic and product purity issues are such that approximately 85% of recycled mixed cullet goes into landfills.
There can typically be up to 10% contaminants in recycled glass material, i.e., 200 lbs. per ton. It would be advantageous if recycled glass could be sorted more effectively to remove non-glass materials. This problem is acute with respect to ceramics, pyroceramics and tempered glass, which resemble glass in many of their properties.
In addition to avoiding waste of material, glass cullet liquifies at a lower temperature than new glass batch, and has favorable viscosity characteristics. Less heat energy is necessary to melt cullet than new batch, reducing costs and environmental emissions. Processing time is reduced, improving productivity. It would be advantageous to use recycled glass to make new glass containers, fiberglass and other products in order to conserve resources and reduce costs.
For all these reasons, it would be advantageous to provide a more practical technique to distinguish and sort glass and non-glass materials having properties similar to glass, which can be operated on a production scale. The present invention provides a method and means for distinguishing among various contaminant materials by taking advantage of their different thermal characteristics with respect to rates of heating and/or cooling and their distinct dielectric strengths and loss tangents, which produce different temperatures when subjected to the same electromagnetic radiation and handling conditions. The invention uses a mass heating technique to maximize temperature differences in a particulate stream, redistributes the heated particulates, and analyzes their thermal image in distinct sections, for rendering hot and/or cool spots detectable and easily diverted from the stream.